1. Field of the Invention
The present invention relates to an internal combustion engine.
2. Description of the Prior Art
There are known air-cooled engines having partly spherical combustion chambers which house the valve heads of intake and exhaust valves that are movably supported in the cylinder head.
If the intake and exhaust valves are positioned closely to each other, then they tend to disturb a flow of cooling air applied to the engine. Therefore, the flow of cooling air cannot efficiently cool the intake and exhaust valves and a spark plug that is located near the intake and exhaust valves.
Japanese laid-open patent publication No. 53-11205 discloses an engine having a partly spherical combustion chamber. The disclosed engine also has an intake valve, an exhaust valve, and a spark plug that face into the combustion chamber. The intake and exhaust valves are inclined at the same angle with respect to the axis of the cylinder bore.
In the disclosed engine, however, the spark plug is mounted in the cylinder head at an angle different from the angle of the intake and exhaust valves. Therefore, holes for receiving the intake valve, the exhaust valve, and the spark plug have to be defined in the cylinder head at different angles. To form such holes in the cylinder head with a drilling machine, it is necessary to change the angle of the cylinder head with respect to the drill bit or the angle of the drill bit with respect to the cylinder head each time a cylinder head is to be drilled by the drill bit. Consequently, it has been difficult, time-consuming, and costly to manufacture the disclosed engine.